Valves are used in a variety of industries to control the flow of fluids. In particular, gate valves are used extensively in the oil and gas industry to control the flow of produced fluids at various stages of production. Most gate valves used in this industry comprise a valve body having a longitudinal flow bore and a transverse gate cavity that intersects the flow bore. A gate having a gate opening extending transversely therethrough is disposed in the gate cavity. A valve stem is provided for moving the gate between an open position, in which the gate opening is aligned with the flow bore, and a closed position, in which the gate opening is offset from the flow bore. The gate cavity of the valve body is covered by a bonnet having an axial bore transverse to the flow through which passes the valve stem.
Such a gate valve is usually associated to a valve operator assembly for selectively driving the valve stem up and down in order to close and open the gate valve. The valve operator assembly generally comprises a transmission mechanism to convert the rotational motion of a drive input into axial motion of the valve stem. To quickly open and close the gate valve with a minimum number of turns, the transmission mechanism may be a ball screw mechanism or a planetary roller screw mechanism in order to reduce the operating torque, for instance manual hand-wheel torque or powered with electric, hydraulic or pneumatic drive for surface valves or with remote operating vehicle (ROV) torque tool or electric or hydraulic actuation for subsea valves. For more details, it is possible for example to refer to the patent EP-B1-1 419 334 (SKF).
When operating a gate valve, the force to apply to the valve stem is usually maximum at the beginning of the valve opening and also at the end of the valve closing. Then the force to apply is less important and substantially constant. Accordingly, the required operating torque on the rotational wheel, such as a manual hand-wheel, is usually much higher at the start of the valve opening and at the end of the valve closing.
One aim of the present invention is to overcome this drawback.